1. Field of the Invention
Embodiments of the present invention relate to an angle detection device that detects a rotational angle of a rotor of a motor, a motor driving device that rotationally drives the motor including the angle detection device, and an image forming apparatus.
2. Description of the Related Art
When rotational position control is performed for a motor, a rotational angle of a rotor is detected. In general, the rotational angle of the rotor of the motor can be detected by connecting a rotary encoder to a rotational shaft of the rotor of the motor. In this case, two-phase pulse signals are obtained from the rotary encoder. A phase difference between the two-phase pulse signals is a 1/4 period. The two-phase signals vary depending on the rotational angle of the rotor of the motor. The relative rotational angle of the rotor of the motor is detected based on a result of detecting edges of the two-phase pulse signals and a High/Low state which has two phases.
An optical encoder is an example of the rotary encoder. The optical encoder includes a disk and two photo interrupters. In the disk, slits, which serve as optical windows, are provided at a periphery while the slits are evenly spaced apart. The two photo interrupters are separately arranged by an interval which is a quarter of the slit pitch of the disk. The two-phase pulse signals are obtained by binarizing output signals from the two photo interrupters.
In the invention disclosed in Patent Document 1 (Japanese Registered Patent No. 4111813), two magnetic sensors are disposed. The two magnetic sensors output corresponding signals that sinusoidally vary depending on the rotational angle of the rotor of the motor. A phase difference between the signals output from the corresponding two magnetic sensors is 90 degrees. Here, the rotational angle of the rotor of the motor is denoted by θ. Among the two-phase output signals that are obtained from the magnetic sensors, an X-phase signal is denoted by Vx, and a Y-phase signal is denoted by Vy. In this case, the Vx can be represented by a cosine function having an amplitude of Ax with respect to the rotational angle θ, and the Vy can be represented by a sine function having an amplitude of Ay with respect to the rotational angle θ. The cosine function and the sine function are shown, for example, in FIG. 19. They are also expressed by Expression 1 that represents two-phase sine signals. In general, the amplitudes Ax and Ay of the signals output from the corresponding magnetic sensors are different. However, the amplitudes Ax and Ay have been adjusted in advance, and it is assumed that Ax=Ay.
                    {                                                                              V                  X                                =                                                      A                    X                                    *                                      cos                    ⁡                                          (                      θ                      )                                                                                                                                                                V                  Y                                =                                                      A                    Y                                    *                                      sin                    ⁡                                          (                      θ                      )                                                                                                                              [                  Expression          ⁢                                          ⁢          1                ]            When the values Vx and Vy of the signals output from the corresponding magnetic sensors are detected, an angle between a vector formed by the detected Vx and Vy on the XY plane and the X-axis corresponds to the rotational angle θ of the rotor of the motor. Therefore, the vector (Vx, Vy) is rotated in the negative rotational direction by repeatedly applying a rotational transform expressed by Expression 2, until the y-component of the rotated vector Vy′(n) changes to a negative value from a positive value. As shown in FIG. 20, each time the rotational transform is applied, the vector is rotated by a step angle of “θstep.” The rotational transform expressed by Expression 2 shows an angle search algorithm. The total rotational angle θn of the thus obtained vector is the detected value of the rotational angle of the rotor of the motor. By periodically performing the above-described operations, the rotational angle of the rotor of the motor can be detected.
                                          [                                                                                                      V                      X                      ′                                        ⁡                                          (                      n                      )                                                                                                                                                              V                      Y                      ′                                        ⁡                                          (                      n                      )                                                                                            ]                    =                                    [                                                                                          cos                      ⁢                                                                                          ⁢                                              θ                        n                                                                                                                        sin                      ⁢                                                                                          ⁢                                              θ                        n                                                                                                                                                                                -                        sin                                            ⁢                                                                                          ⁢                                              θ                        n                                                                                                                        cos                      ⁢                                                                                          ⁢                                              θ                        n                                                                                                        ]                        ⁡                          [                                                                                                                  V                        X                        ′                                            ⁡                                              (                                                  n                          -                          1                                                )                                                                                                                                                                                V                        Y                        ′                                            ⁡                                              (                                                  n                          -                          1                                                )                                                                                                        ]                                      ⁢                                  ⁢                                                            V                X                ′                            ⁡                              (                0                )                                      =                          V              X                                ,                                          ⁢                                                    V                Y                ′                            ⁡                              (                0                )                                      =                                                            V                  Y                                ⁢                                                                  ⁢                                  θ                  n                                            =                              n                *                                                      θ                    step                                    ⁢                                                                          (                                                            n                      =                      1                                        ,                    2                    ,                    3                    ,                    …                                    ⁢                                                                          )                                                                                        [                  Expression          ⁢                                          ⁢          2                ]            However, for the optical encoder, the components such as the slit disk and the photo interrupters are utilized. Further, these components are assembled. Consequently, the cost is increased.
Further, in the method described in Patent Document 1, for example, a brushless motor includes plural Hall elements that are arranged to output signals that are evenly shifted by a predetermined phase, so as to detect timing to switch the current. However, in the most commonly used three-phase brushless motor, the Hall elements for switching the current are arranged so that output signals are evenly shifted by a phase of 120 degrees. In such a case, in order to detect the rotational angle of the rotor of the motor, Hall elements may be added so as to output signals that are evenly shifted by a phase of 90 degrees. In a general-purpose motor unit, the coils, the rotor, and the Hall elements, which are arranged to output the signals that are evenly shifted by the phase of 120 degrees, are integrally assembled. Therefore, such a general-purpose motor unit may not be used, and consequently the cost may be increased.
An objective of the embodiments of the present invention is to provide an angle detection device that can stably and accurately detect an angle of a rotor of a motor, without an increase of cost, such as due to additional components.